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Chapter 3 Atoms and Elements
Atomic Theory
Atoms are tiny particles of matter.
Atoms are made up of subatomic particles:
protons, neutrons and electrons
.
Protons have a positive (+) charge.
Electrons have a negative (–) charge.
Neutrons are neutral.
Like charges repel, and unlike
charges attract.
size of atom/size of nucleus = 100000/1
roughly a football to a football field in
area
Atomic diameter 10-10
(p)
1.672*10-24 g
mass of a neutron (n)
1.674*10-24 g
mass of an electron (e)
9.1*10-28 g
mass of a proton
mass p/e = 1.672*10-24/9.1*10-28 = 1840/1
Chapter 3 Element and Symbols
copper Cu
Elements are pure substances that cannot be separated into simpler substances by
ordinary laboratory processes; they are the building blocks of matter
gold
aluminum
carbon
Au
Al
C
Many elements can be found in different forms; The arrangement in of the atoms are
different in these different forms; diamond and graphite (lead in your pencil) are different
forms of carbon
Do I need to
know the
symbols of all
these elements?
H
Li
Na
K
Mg
Ca
B
C
N
hydrogen
lithium
sodium
potassium
magnesium
calcium
boron
carbon
nitrogen
He helium
O oxygen
F
fluorine
Al aluminum
P phosphorus
S
sulfur
Cl chlorine
Br bromine
I iodine
Cr
Mn
Fe
Ni
Cu
Zn
chromium
manganese
iron
nickel
copper
zinc
The heavy zigzag line separates metals and nonmetals.
Metals are located to the left.
Nonmetals are located to the right.
Metalloids are located along the heavy zigzag line between the metals and nonmetals.
Why is the distinction made between metals and non-metals?
Metals are generally found combined with non-metals in the compounds they form
Halogens
Alkali metals
Fay:alkali metals
Fay: halogens
Metals are shiny and ductile; are good conductors of heat and electricity
Nonmetals are dull, brittle, and poor conductors of heat and electricity;are good
insulators
Metalloids are better conductors than nonmetals, but not as good as metals; are used
as semiconductors and insulators
Atomic number and mass number
sulfur
16S
32
The atomic number is specific for each element;
is the same for all atoms of an element
is equal to the number of protons in an atom
The mass number represents the number of
particles in the nucleus;
is equal to the sum of the number of
protons and number of neutrons
sulfur
16S
32
32S
16
Atoms of the same element
always have the same atomic
number but may have different
atomic masses; the mass number
is always equal or larger than
the atomic number
3Li
7
6C
12
3Li
6
6C
13
6C
14
Isotopes and atomic mass (weight)
Isotopes are atoms of the same element that have different mass numbers;
they have the same number of protons but different numbers of neutrons
Isotopes are difficult to separate from each other, therefore they occur in any sample of the
element in their natural abundance which remains relatively constant, regardless
of the source of the element.
Most but not all isotopes are stable. Those that are unstable are referred to as being
radioactive.
This is what is found on a periodic table for Cl
How was an atomic mass value of 35.45 arrived at?
Since in a “handful” of Cl there is a mixture of two isotopes
in the abundances shown on the left, an average atomic
mass has been defined
Average Atomic Mass Cl = 0.7576(Cl35) + 0.2434(Cl37)
= 0.7576(35) + 0.2434(37)
=
35.45
Since most elements in the periodic table have stable isotopes,
most atomic masses are not whole numbers but rather averages
of the atomic masses of their isotopes adjusted for their natural
abundances
An exception is 9F19 which consists of only one isotope
Which of the isotopes listed above for each element do you think is most
abundant?
Most of the isotopes listed above are stable; carbon 14 is an exception
A model of the atom
The electrons (e-) are
blown up so that you
can see them
3Li
7
The electrons which are negatively charged are believed to be distributed around the
nucleus since it is possible to remove an electron by various means and form new species
which are positively charged. Futhermore, there are experiments that suggest that most of
the mass of the atom is located in only a very small portion of the atom and that most of the
atom is relatively empty space. Remember that the size of atom/size of nucleus = 100000/1
Light: What is it?
Everything we know about our universe comes from the study of how light interacts with matter
Light is referred to as electromagnetic radiation
Light has both wave properties and particle properties
All light, whether radiowaves or visible light, travels as the same speed, 3 *108 meters/sec
As a result, since the length of each wave decreases from left to right, the frequency of the
peaks an troughs of the waves shown above must increase from left to right
Referring to light as a particle, known as a photon of light, the energy of each particle of
light is also known to increase from left to right
The total energy of course depends on the frequency of the light and the number of particles
Light also has a magnetic field associated with. If varies in the same fashion as
the electric field, traveling at the same speed but perpendicular to the electric
field. Both the electric and magnetic fields are used in medicine
A MRI (magnetic resonance image ) of a
heart and lungs using radiowave
frequencies in the presence of a strong
external magnetic field
White light that passes through a prism
is separated into all colors that together are
called a continuous spectrum gives the colors
of a rainbow
When an element is heated, it gives off
light. However the entire rainbow of
colors is absent and only certain colors are
present. Each element gives it own
spectrum of color. Not all of the light is in
the visible region. Depending on the
temperature, the element, some light
covering a large portion of the
electomagnetic spectrum can be observed
A model has been devised to explain this phenomena
Think of the model as a bookcase with each
succeeding shelf getting closer and closer
Depending on the element, different electronic
transitions can be observed. The emphasis her is
electronic, the electrons are being excited to
different levels
An e-
The periodic table is named periodic
because it has been know for a long
time that many of the chemical
properties of the elements are very
similar but very different from
others. This is why they are lumped
into groups.
The theory that has been developed
therefore, has been developed to
explain this periodicity. Since it is
mathematical in origin, we will
simply look at it as a model.
2d
2p
2s
2p
2s
1s
Also on closer inspection of the different n levels, additional fine structure is observed
within each n level and these are assigned different letters of the alphabet.
According to the mathematics each s, p, d level can accommodate 2 electrons. There is one
s level for each shelf, three equivalent p levels for shelf 2 and above and 5 equivalent d
levels for the third shelf and above. The difference between the three equivalent 2p levels
is their arrangement in space. Otherwise they are equivalent in shape. Similarly for the d
levels.
These orbitals according to theory is the region in space one is
likely to encounter an electron; volumes of highest probability
1H
3Li
7N
2He
4Be
8O
6C
9F
5B
10Ne
11Na
valence electrons determine the chemical properties of the elements;
they are the electrons in the outermost, highest energy level
and are related to the Group number of the element
How many valence electrons does this neutral atom have and what is the element?
A summary of the periodic table and electron orbitals